A radioreceptor assay using [3H]2-beta-endorphin (B-EP) was used to examine binding in the brains of rats that had been exercised on a treadmill for 5 months. The binding of [3H]2-beta-EP to brain homogenates gave an average Kd of 0.830 +/- 0.055 nM for the controls and 0.826 + 0.022 nM for the runners, and respective Bmax's of 166 +/- 20 pM and 159 +/- 27 pM.
